Start your fort one by one (1x1). The moster slayers will have easier and shorter time traveling trough caverns and most important thing of all, slaying monsters together which will help them survive for longer and thus will train their monster slaying skill for longer (combat skills).
Do not open up caverns imidietly, instead wait for them to arive in greater numbers (10+ should be enough I guess).

When you have suficient amount of monster slayers symply open up caverns for them and they will start to do their job.
Foreign (non dwarf) monster slayers carry exotic weapons like whips, halberds, two handed swords, pikes, scimitars and other stuff that you can not produce on your own, thus in the case they die you can use these weapons in the military.
Human monster slayers also carry clothes and armour too large for your dwarfes, so if they die either smelt the gear and make some good fiting stuff for your dwarfes or just sell it to traders.
Monster hunters with ranged weapons should be considered with comparison to mele weapons, since they usualy be in colse proximity to their targets they will simply proceed to bludgeon them with their ranged weapons, they will also do it automatically when they have no ammo.

The dead monster slayers are a good source of smelting gear and weapons, only one major downside is that they require burial and burial rights as any other citizen of your fortress, so be wary of potentional ghosts.

The ground wariant of them are beast slayers (so far I have not seen any beast slayers, might be biome dependant, compared to monster slayers they are far less common, not sure what triggers them to show up or if they are even real), but this guide is about monster slayers, which are more common and start to show up once you discover underground caverns.

Now go and strike the earth (and harras/bully the local underground wildlife too).

The end of the guide.